Material bed roller mills are quite generally known for instance from DE-C No. 27 08 053 and facilitate a considerable saving of energy in the comminution under pressure of brittle material for grinding, such as for example cement clinker, ores etc. In such material bed roller mills two driven rollers which are arranged horizontally adjacent to one another are pressed against one another at high pressure. When the material passes through the grinding gap between the two rollers it is comminuted to a large extent, and agglomerates (lumps) are formed which contain a high proportion of fine or finished material and are then broken up in a subsequent machine with a low energy consumption.
Of the two rollers of such a material bed roller mill one roller is constructed as a fixed roller which is mounted stationary and the other is constructed as a floating roller which is mounted so as to be movable at right angles to the grinding gap, and the floating roller is sprung and pressed at high pressure in the direction of the fixed roller in order to bring about the comminution under pressure of the material for grinding as explained above. In order to produce this high pressure it is known in the art for a combined hydraulic/pneumatic spring system to be arranged on each side of the floating roller, in which at least one hydraulic working cylinder which co-operates with a working pneumatic spring is provided for each side of the floating roller, i.e. for each axle neck of the floating roller. With the aid of these hydraulic/pneumatic spring systems the optimum grinding force should be set in the grinding gap by means of the floating roller and the desired pneumatic spring characteristic (rigidity of the spring) should also be set.
In this construction which is known in the art the optimum setting particularly of the grinding force is achieved above all by a number of tests in which different settings for the preliminary filling pressures of the gas and hydraulic fluid are tried, and each time the roller mill has to be switched off and a new preliminary filling pressure set. During the setting operation the working pneumatic spring which is present in each hydraulic/pneumatic spring system is first of all filled with gas to a predetermined preliminary gas filling pressure, and then the hydraulic oil is fed in to a predetermined preliminary filling level in order to set the grinding force with a so-called zero gap, that is to say when the flux of force between the two rollers runs over a plurality of spacers by means of which a minimum roller spacing is maintained. The pneumatic spring characteristic and the grinding force behaviour during the comminution of the material for grinding are determined by the preliminary gas and oil filling pressures. In the known material bed roller mills which are constructed in this way it is not possible to adjust the pressure in the hydraulic-pneumatic spring system directly in case of need (for example by delivery and removal of hydraulic oil) especially as the spring characteristics of the pneumatic springs on both sides of the floating roller should be identical after the adjustment, and therefore it is always necessary to switch off the roller mill for each adjustment and then restart it.
The object of the invention, therefore, is to make further developments to a material bed roller mill in such a way that using relatively simple measures the grinding force can be adjusted in a simple manner, even during mill operation, and thus can be adapted to changed comminution conditions.